1. Field of the Invention
The present invention relates to an illumination optical system and a projection display optical system which are used in a projection display apparatus or the like.
2. Description of Related Art
Conventionally, in a projector type display (a projection display apparatus), a liquid crystal display panel or a micromirror array device panel is typically used as a light modulation element for switching to control transmission and shielding or deflection of light to project a selected light pattern onto a screen, thereby displaying an image on the screen.
In the projector which employs the liquid crystal display panel or the micromirror array device panel as the light modulation element, it is important to use light from a light source with high efficiency and reduce variations in illuminance on the screen.
An optical integrator formed of two lens arrays each including lenses arranged two-dimensionally is a known means for improvement. In the optical integrator, a first lens array splits a luminous flux from a light source into a plurality of luminous fluxes, and a second lens array and a condenser lens enlarge the luminous fluxes and form images by the luminous fluxes superimposed one on another on a display area of a light modulation element (see Japanese Patent Application Laid-Open No. 11(1999)-64848).
In this method, since the split luminous fluxes with small variations in illuminance are superimposed, the resulting irradiation light has high uniformity to significantly reduce variations in illuminance on the screen. When the first lens array has each aperture formed in a rectangular shape similar to the display area of the light modulation element, all the split luminous fluxes are irradiated to the display area without waste. This improves the efficiency of the irradiation light and thus improves the use efficiency of the light from the light source.
Another means for improvement is to guide light from a light source to a kaleidoscope to mix the vectors of light rays to provide uniform light intensity distribution at an end surface of the kaleidoscope from which the light emerges, and then form a conjugate image by an image-forming lens on a micromirror array device used as a light modulation element.
When the kaleidoscope is used, an optical system is complicated if a means for converting natural emission light from the light source into linearly polarized light is used. Thus, such a means is not used generally.
In the method, the resulting irradiation light has high uniformity to significantly reduce variations in illuminance on a screen.
However, in the method of providing uniform light intensity distribution using the optical integrator formed of two lens arrays or the kaleidoscope, the luminous flux illuminating the light modulation element has a large convergent angle. When the light modulation panel is realized by a reflection type liquid crystal display panel or a micromirror array device, limitations are imposed on space for forming an optical path along which the illumination light is guided. When a TIR (total internal reflection tilt) prism is used to guide light, the minimum angle of total reflection is limited. When a polarization beam splitter is used to guide light, limitations are imposed due to dependency of the reflectivity of S waves and transmittance of P waves on the incident angle. From these facts, the illumination luminous flux incident on the light modulation element is desirably close to a collimated luminous flux.
In addition, when a transmission type liquid crystal display panel is used as the light modulation element to modulate light of tree primary colors of red, green, and blue, the modulated light components are then combined by a dichroic mirror or dichroic prism. In this case, as the modulated light is less similar to a collimated luminous flux, the cut wavelength in a reflection/transmission wavelength region of a dichroic film is changed to produce turbidity of colors or variations in color reproducibility depending on the position of a projected image.
When twisted nematic liquid crystal (TNLC) is used as the light modulation element, whether it is of a transmission type or a reflection type, as the incident angle of an illumination luminous flux on the liquid crystal display panel is more inclined with respect to the normal to the panel, and generally, more inclined with respect to each side direction in the liquid crystal display panel plane, a larger deviation occurs from 0 or π which is an ideal phase difference of a wave provided by transmission through the liquid crystal display panel. Therefore, contrast in light modulation is reduced.
To address this, the present inventors have proposed an illumination optical system which illuminates an illumination surface with a generally telecentric illumination luminous flux (which means that it includes somewhat divergent and convergent components) in which, in intensity distribution of illumination light on the illumination surface changing depending on a deviation angle of an incident ray with respect to a normal to the illumination surface, a ratio of angle widths at which light intensity reaches half of a peak value in each of two axis directions orthogonal to each other on the illumination surface is an aspect ratio of 2:1 or higher. The illumination optical system has an optical integrator which performs splitting and recombination on the luminous flux in a first axis direction on a section generally orthogonal to the traveling direction of the illumination luminous flux, and a light intensity conversion element which performs conversion of light intensity distribution in a second axis direction orthogonal to the first axis direction on the section.
This can realize an illumination optical system which can use light from a light source with high efficiency and can provide an illumination luminous flux with highly uniform illuminance. The illumination optical system can be used as an illumination section in a projection display optical system to provide a projected image with a high contrast.
In the illumination optical system described above, however, when an incident luminous flux from a light source lamp has large divergence, the use efficiency of light from the light source may be reduced.
In commercially available full-color projection type display apparatuses, a color splitting/recombination optical system typically has a color splitting direction set to a horizontal direction of a projected image. This is because an apparatus in an oblong shape is conveniently handled as a video-related device. For image signals, a displayed image has a length-to-width ratio of 4:3 as an image display ratio in the NTSC system, or a length-to-width ratio of 16:9 as an image ratio in the MUSE system. A long side direction of a light modulation panel naturally matches the direction of color splitting/recombination for full-color display.
In other words, the direction of luminous flux splitting in a wavelength band splitting film (a dichroic mirror or the like) or a polarization beam splitter corresponds to the long side direction of the light modulation panel.
To prevent degraded accuracy of light splitting due to variations in an incident angle of light on the wavelength band splitting film or the polarization beam splitter, it is necessary to set a small incident angle of an illumination luminous flux on the light demodulation panel in the direction of color splitting/recombination, that is, the long side direction of the light modulation panel.
Thus, in the illumination optical system proposed by the present inventors described above, the direction of optical integration is set to a short side direction of a light modulation panel.
In this case, a disadvantage occurs in the use efficiency of light from the light source lamp. Specifically, the direction of optical integration is set to the short side direction of the light modulation panel, so that the direction of multi-stage arrangement of a polarization conversion element called a PS conversion element which is mainly formed of polarization beam splitters arranged in multiple stages and half-wave plates is set to the short side direction of the light modulation panel. For this reason, the arrangement directions of the light source and a multi-stage slit mask disposed in the polarization conversion element, generally disposed in the long side direction of the light modulation panel, are also set to the short side direction of the light modulation panel. Consequently, a larger amount of light with divergence from the light source is shielded by the multi-stage slit mask to reduce light transfer efficiency of the illumination optical system.